It is often desirable to control the rate at which a fluid, such as a gas, is delivered to an intended destination. For example, gas precursors can be used as doping sources in an ultra-high vacuum molecular beampitaxy technique for producing semiconductor device structures. Because dopant concentrations are directly related to the flow rate of the gas precursors, control of such flow rate is necessary. Furthermore, in some applications, the flow rate of a fluid may need to be varied over a broad dynamic range of several orders of magnitude. Referring again to the example of an ultra-high vacuum molecular beampitaxy process, the flow of gas precursors may be varied over three orders of magnitude during processing in order to produce dopant concentrations ranging from 10.sup.16 to 10.sup.19 cm.sup.-3.
According to one prior technique, control of gas flow was achieved by diluting a precursor gas with an inert gas. Such technique, however, is not desirable with an ultra-high vacuum molecular beamepitaxy system because the addition of an inert gas increases the gas load on the vacuum pumping system. Consequently, in other prior techniques, temperature-controlled or pressure-controlled mass flow controllers were used to vary the flow rate of a gas. These previously developed mass flow controllers, however, suffered from numerous problems. For example, such flow controllers included complex electronics which needed to be recalibrated often. Also, these flow controllers employed mechanical valves which extended from the outside of the controllers into the interior. Fluid could escape from the controllers at such mechanical valves. Yet another problem was that the previously developed flow controllers operated within a limited dynamic range of flow rates, and thus were incapable of varying gas flow over several orders of magnitude. Furthermore, specific flow rates could not be consistently reproduced using such mass flow controllers.